The present invention relates to digital images. More specifically, the present invention relates to compression of color document images.
Compound color documents may contain text, drawings and photo regions (sometimes overlaid), complex backgrounds (e.g., text boxes), watermarks and gradients. For example, magazines, journals and textbooks usually contain two or more of these features.
A page of a magazine may be captured by an image capture device (e.g., a scanner) and the resulting digital image of the page may be stored on a computer hard drive as an electronic file. The electronic file can be rather large. This makes it desirable to compress the digital image into a smaller file that takes less space to store.
Compression standards such as JPEG are suitable for compressing photo regions of the compound color documents. However, compression standards such as JPEG are usually not suitable for compressing text regions.
Compression standards such as CCITT, G4 and JBIG are suitable for compressing black and white text regions of the compound color documents. However, compression standards such as CCITT, G4 and JBIG are usually not suitable for compressing the photo regions.
According to one aspect of the present invention, compression of a digital image is performed by truncating bit depth of a luminance channel of the digital image and performing lossless compression on the truncated luminance channel; and performing lossy compression on chrominance channels of the digital image.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention.